The present invention relates to sulfide and oxysulfide pigments which are based on platelet-shaped substrates coated with a sulfide or oxysulfide of a rare earth metal and/or of yttrium, and to their preparation and use, especially in car finishes, paints, inks, plastics and cosmetic formulations.
Rare earth metal sulfide and yttrium sulfide pigments without carriers, and their use in plastics, coating materials and cosmetics, are known from EP 0 203 838 B1. EP 0 545 746 B1 describes pigments comprising both a composition based on a sesquisulfide of a rare earth metal or of yttrium and a dopant.
In EP 0 620 254 B1 a pigment based on a rare earth metal sulfide or on yttrium sulfide is used as a carrier which is coated with a transparent oxide, preferably a metal oxide.
These known rare earth metal sulfide and yttrium sulfide pigments, although they can be employed to good effect as absorption pigments for numerous purposes, cannot be regarded as pearl lustre pigments owing to the absence of interference capacity and to deficient lustre.
It was therefore the object of the invention to provide rare earth metal sulfide or oxysulfide and yttrium sulfide or oxysulfide pigments having good opacity and strong colour shades, covering a relatively broad colour spectrum, and at the same time having the interference and lustre properties of pearl lustre pigments.
It has surprisingly now been found that, by coating platelet-shaped substrates with sulfides or oxysulfides of the rare earth metals (lanthanides) and of yttrium, pigments having advantageous coloristic properties are obtained.
On the platelet-shaped substrates it is possible to apply uniform sulfide or oxysulfide layers of rare earth metals and of yttrium which have a high lustre, give the pigment an attractive powder colour, and may possibly generate the interference colours of thin platelets.
The invention therefore provides sulfide and oxysulfide pigments which are based on platelet-shaped substrates, characterized in that the substrates are coated with a sulfide or oxysulfide of a rare earth metal and/or of yttrium.
The invention additionally provides a process for preparing the pigments of the invention and for the use of these pigments in formulations such as paints, varnishes, printing inks, plastics and cosmetics.
The substrates are coated with a sulfide of the rare earth metals or of yttrium having the formula M2S3, in which M is at least one element chosen from the group formed by the lanthanides of atomic numbers from 57 to 71 inclusive and yttrium. Preferably, M2S3 is a sesquisulfide of cubic xcex3-Ce2S3 or xcex3-La2S3. However, sulfide mixtures, such as xcex3-Ce2S3 and xcex3-La2S3, or mixed sulfides, such as LaYS3, are also suitable. The coatings may additionally consist of oxysulfides of the formula M2S3xe2x88x92xOx (2.5 less than xc3x97 less than 0.05)
A suitable platelet-shaped base substrate is any platelet-shaped material known to the person skilled in the art which is stable under the conditions of coating. Particular mention may be made here of natural and synthetic mica, kaolin, talc, vermiculite, TiO2, SiO2 and Al2C3 flakes, glass flakes, graphite, metal flakes, bismuth oxychloride, platelet-shaped iron oxide, LCPs (liquid crystal polymer pigments), holo-graphic pigments and other platelet-shaped materials.
Preference is given to mica, such as muscovite or phlogopite, for example. The platelet-shaped substrate employed can, however, also comprise materials which already possess a metal oxide coating, especially mica or SiO2 flakes with one or more coats of, for example, TiO2, ZrO2, SnO2, Al2O3, SiO2, ZnO or mixtures of these metal oxides.
Other suitable substrate materials are thin metal flakes, which in some casesxe2x80x94as in the case of Al flakes, for examplexe2x80x94are enveloped by a layer of a stabilizing compound of low refractive index, such as silica. Apart from Al flakes, other metal flakes employed include those of Ag, Ti, Cu/Zn and other metals.
The size of these platelet-shaped substrates is not critical per se, and it is therefore possible to use particles of the size appropriate for the intended application. In general, the substrate will be employed in particle sizes of from about 1 to 200 xcexcm, in particular from about 5 to 100 xcexcm. The thickness of the particles is generally from about 0.1 to 5 xcexcm, in particular about 0.5 xcexcm.
The starting materials used as substrates are known and can be prepared by known techniques. Mica particles of the desired size order can be obtained by wet or dry milling of mica followed by classification. Metal oxide-coated materials, especially metal oxide-coated mica flakes, can either be obtained commerciallyxe2x80x94for example, as Iriodin(copyright) pearl lustre pigment from Merck KGaA, Darmstadt, Germanyxe2x80x94or can be prepared by known techniques. Such techniques are described, for example, in the following patents and patent applications: U.S. Pat. No. 3,087,828, U.S. Pat. No. 3,087,829, DE 19 59 998, DE 20 09 566, DE 22 14 545, DE 22 44 298, DE 23 13 331, DE 25 22 572, DE 31 37 808, DE 31 37 809, DE 31 51 343, DE 31 51 354, DE 31 51 355, DE 32 11 602, DE 32 35 107, WO 93/08237 and EP 0 763 573.
The pigments of the invention are prepared by adding a salt solution of a rare earth metal and/or of yttrium and optionally oxalic acid to an aqueous suspension of a platelet-shaped substrate, the pH of the suspension being optionally kept substantially constant, by simultaneous addition of a base, within a range which brings about hydrolysis of the added salt, and the pigment coated in this way with an oxide or oxide hydrate or oxalate is separated off, washed, dried, preferably at 80-150xc2x0 C. for from 10 minutes to 1.5 hours, and calcined, preferably at 400-900xc2x0 C. Finally, the product is calcined in an H2S gas stream or in a CS2 or H2S/CS2 gas stream under inert gas for from 10 minutes to 2 hours at temperatures from 550 to 1200xc2x0 C., preferably from 800 to 1000xc2x0 C., in the course of which the oxalate or oxide of the rare earth metal and/or yttrium is converted to the sulfide or oxysulfide.
It is also possible to carry out the reaction with H2S or CS2 in a fluidized-bed reactor (CVD process).
In order to conduct the CVD variant it is advisable, as is generally the case for CVD processes, to use a fluidized-bed reactor or tube furnace. The substrate particles to be coated are heated in the reactor, for example with fluidization by means of an inert fluidizing gas such as N2 or argon, to the desired reaction temperature (generally from 400 to 900xc2x0 C., preferably from 650 to 850xc2x0 C.). The elemental sulfur or H2S and/or CS2 is then introduced from separate nozzles with the aid of inert streams of carrier gas (advantageously substreams of the fluidizing gas) from upstream evaporator vessels, the sulfur concentration being held judiciously at from 0.5 to 5% by volume, preferablyxe2x89xa62% by volume, based on the total amount of gas in the reactor.
In addition to compounds containing sulfur, particularly preferred sulfur donors include hydrogen sulfide and elemental sulfur.
If elemental sulfur is used, the procedure adopted is judiciously to render finely ground sulfur powder inert for about 1 to 4 h and then to heat it in the absence of oxygen to the reaction temperature, generally 400-1200xc2x0 C., preferably 400-900xc2x0 C. and, in particular, 600-850xc2x0 C.
By depositing rare earth metal oxide or oxide hydrate layers or yttrium oxide or oxide hydrate layers in the presence of one or more dopants, selected, for example, from the group of alkaline earth metals and/or alkali metals, onto the platelet-shaped substrates, it is possible to produce particularly smooth, stable, defined and strongly coloured rare earth metal sulfide layers or yttrium sulfide layers, respectively. The dopant is in the form of an inclusion in the crystalline lattice of the sulfide layer M2S3 or the oxysulfide layer M2S3xe2x88x92xOx. The dopant can be selected alone or in mixtures from the salts of alkali metals and/or alkaline earth metals; preference is given to the use of sodium or potassium. In the course of coating the platelet-shaped substrates with the oxide or oxide hydrate layer, the dopant is added in solid form or in aqueous solution in amounts, based on the substrate, of from 0.02 to 2.0% by weight, preferably from 0.05 to 0.2% by weight. The proportion of dopant in the sulfide layer or the oxysulfide layer is from 0.01 to 1.0% by weight, preferably from 0.05 to 0.5% by weight.
Particular preference is given to platelet-shaped substrates which have a cerium sulfide layer or an oxysulfide layer and are doped with sodium.
Depending on the desired effect, the sulfide layer or oxysulfide layer of the rare earth metals or of the yttrium can have thicknesses of up to about 400 nm, preferably from 10 to 300 nm. In these layers it is common to achieve contents of rare earth metal sulfides or of yttrium sulfides or of the corresponding oxysulfides which, based on the substrate, are from about 1 to 400% by weight, in particular from 5 to 300% by weight. Depending on the layer thickness of the sulfide or oxysulfide layer, interference colours are obtained which pass, with growing layer thickness, from silver via gold, red, violet and blue through to green and, finally, to higher-order interference colours.
With pigments containing not only a rare earth metal sulfide or yttrium sulfide layer or a corresponding oxysulfide layer, but also other layers (preferably 3 to 7 layers) of materials of high and low refractive index (preferably oxides) and, optionally, semitransparent metals (multilayer pigments) it is possible to obtain very strong angle-dependent colour effects (colour flop, goniochromaticity).
It is also possible to subject the finished pigment to an aftercoating or aftertreatment operation which further enhances the light stability, weathering stability and chemical stability or facilitates the handling of the pigment, especially its incorporation into various media. Examples of suitable aftercoatings and aftertreatments are the processes described in DE-C 22 15 191, DE-A 31 51 354, DE-A 32 35 017 or DE-A 33 34 598.
The additionally applied substances make up only about 0.1 to 5% by weight, preferably about 0.5 to 3% by weight, of the overall pigment.
The pigments of the invention are a major enrichment of the art. The powder colour, passing from orange to red, and the interference colour, which can be generated as desired, result in extremely interesting effects which can be exploited for numerous applications; in the case, in particular, of substrates which already themselves have an interference colour, this can be intensified and varied by means of the sulfide layer or oxysulfide layer. Areas of application arise both in cosmetics, where the pigments of the invention can be employed, for example, in nail varnishes, in make up, gels, powders, ointments, emulsions, lipsticks and other formulations, in concentrations of generally from 0.1 to 80% by weight, and in industry, for pigmentation of paints, other coating materials, printing inks or plastics, for example.
Plastics comprising the pigments of the invention in amounts of from 0.1 to 10% by weight, preferably from 0.1 to 5% by weight, are outstandingly suitable for laser marking using, for example, a CO2 or Nd-YAG laser. The markings obtained are notable for their high contrast and definition.